1. Field of the Invention
The present invention relates to an optical waveguide having the function of changing the direction of light propagation.
2. Description of the Related Art
Since the technology of high-speed transmission of signals using electricity is approaching its limit, there are great expectations in the role of optical transmission. In this situation, the realization of an opto-electric hybrid board is regarded as a today's urgent task. In order to realize the opto-electric hybrid board, an optical waveguide corresponding to a highly integrated electrical device is required. It is required for the optical waveguide to attain a large change in the direction of light propagation in a small space within limits imposed by the integration. A polymer waveguide has a higher degree of design freedom than a waveguide formed of quartz materials. Although methods of changing the direction of light propagation using various polymer waveguides are being considered, there are problems such as those described in the following (1) to (3) are imposed.
(1) Problem Relating to Utilizing an Arc
Conventionally, an arc was used for a large change in a direction of light propagation. However, it is not possible to avoid a loss caused by utilization of a radiation mode when an arc is used. In order to reduce this loss, the arc must have a radius of curvature which is larger than a certain value, giving rise to the necessity for securing a large space.
(2) Problem Relating to Utilizing a Total Reflection
The use of total reflection by means of a clad is well known. However, since the difference in refractive index between a core and a clad is small, a large change in direction was not possible. Also, complete total reflection cannot be performed and there is a leakage of light at the total reflecting surface.
(3) Problem Relating to Involving the Production of a Mirror Surface by a Dicing Saw
It has been known widely that an optical waveguide is cut at a 45° angle by using a dicing saw and the cut surface is used as a reflecting mirror to change the direction of a light path. However, it is impossible to carry out cutting locally. Also, because the polymer optical waveguide is actually cut in this cutting process, it is difficult to attain this cutting at a place except for the end surface of the optical waveguide. Also, because accuracy of the dicing position is required, leading to an increase in the number of steps as well bringing about high costs.
Among the aforementioned methods, a method involving measures for solving the above problems is proposed in which an arc is utilized and a clad is provided within a core (for example, Japanese Patent Application Laid-Open (JP-A) No. 9-145943). In this method, a sandbank-clad is inserted in the arcing layer and the optical waveguide of the bent part is divided into plural narrow-width optical waveguides to greatly reduce the leakage of light. By this method, the reduction in leakage of light reduces the optical loss and the curvature radius can be made small; however, a limitation to miniaturization remains as before.
As a method of significantly changing the direction of light propagation in a small area, a method can be considered which involves expanding the conditions of total reflection by a localized use of a cladding material having a large difference in refractive index from the core. As an example of such a method, an air clad is proposed (for example, JP-A No. 2003-207661). In this method, an air clad reflecting layer is provided on the exterior of a core in addition to a 90° refraction layer and light is reflected on the air clad. However, in this method, since the air clad layer is located on the exterior, the manufacturing process is complicated and the air clad cannot be easily produced.
Besides the above examples using an air clad, optical waveguides in which a closed void (air foam cell) is provided on the exterior of a bent part of an optical waveguide core which use the planar interface between the void and the core as a reflecting surface are proposed (for example, JP-A Nos. 11-248951 and 2003-75670). In all of these optical waveguides, voids are also present outside of the core of the optical waveguide and the difference in reflection ratios between the void and the core is utilized to constitute a reflecting surface. In the configuration disclosed in JP-A No. 2003-75670, the air clad is formed by etching, but the surface formed by etching tends to be rough, giving rise to the problem that the reflecting surface has a degraded reflecting efficiency. Further, the equipment cost for etching tends to be higher, and the etching process itself is disadvantageous timewise. Also, because the void is an air cell, the whole end surface of the waveguide is not completely in contact with the void due to the method of depositing clad materials and the planar interface is not a perfectly flat surface, optical loss caused by these reasons is inevitable.
Also, the configurations in JP-A Nos. 11-248951 and 2003-75670 must have the whole core of the optical waveguide as a total reflecting surface, and it is therefore impossible to make a branched waveguide structure.
Also, in widely used methods in which a Y-branch or the like is used in a branched waveguide to divide light into plural branches, an increase in a branched angle is accompanied by an increase in light leakage and it is therefore impossible to branch light at a wide angle.
The fundamental cause of these problems is a limitation in the refractive indices of the core and the clad to be used because the NA becomes defined under the conditions of connections between the optical parts such as a fiber and the like and the optical waveguide. Accordingly, it is conceivable that if a clad, which has a different refractive index from that of the core, can be used locally at a place where a direction of light propagation is significantly changed, a large change in the direction of propagation can be realized.
Also, when a polymer having a degree of freedom is used in designing a waveguide, the refractive index of the polymer used for the clad is limited. Therefore, when designing a waveguide without considering NA, such as in the case of directly connecting the waveguide with an optical transmitting and receiving device, the clad is not allowed to have a refractive index significantly different from that of the core, with the result that the direction of light propagation cannot be significantly changed.